Just a FYI. Have the Hemi and read through some old gasoline threads where a couple people reported getting better gas mileage with 87 octane. Well add me to that list. Have been using 89 octane since I got the car and averaged around 12.5 mpg in city driving (Chicago mind you). About 20.5 on the highway at 70. Well that has been bumped to around 14 mpg city and almost 23 highway on a pretty simple test of merely switching my entire last tank to 87 octane. Haven't noticed any reduced power.

Yep. Noticed the same thing. Around town(suburbs-not city) went from about 13.5 to 14.7, and highway at 65-70 from 22 to 23-24. Agree,haven't noticed any power difference. Havent tried towing w/87 maybe a power difference then.

Hmmm, can't make sense out of it either. Isn't octane the resistance to ignite? You can run more timing (advanced) because it doesn't pre-gnite and ping (i.e. you get a longer, more intense power stroke - which is what she said btw)? By going lower grade maybe it alters the fuel map/timing curve that puts it more in the sweet spot of the way you drive (probably more torque down low in city driving?). What is your mix of driving, how do you know you're getting that in the city vs highway? I get an aggregate reading. I'm guessing you're making long haul comparisons from a full tank? Any change in altitudes or other factors. Were you driving uphill one way and downhill the other, LOL?

Doesn't make sense from a the standpoint of an internal combustion engine, but computers (ECU/PCM/TCM) and their fuel/spark maps can do strange things... . Actually what this tells me is that there is more performance in the engine by going higher grade and having the computer adjust accordingly. But more power should mean more efficiency to a point as well. Maybe a pro-tuner will chime in. Everything you've read so far has been me talking out of my arse

The 87 octane fuel has a higher BTU content (energy rating) than higher octane fuels like 89 or 93. But 87 is more susceptible to detonation under load without a lot of timing pull or throttle reduction to eliminate it which ultimately reduces the engine's power output and efficiency. When under moderate/light loads (highway cruising), the engine is not under high output demand (like towing), so an 87 octane fuel can provide better fuel economy than an 89 of 93 as long as the timing pull is not occuring too dramatically under those conditions. I wouldn't expect to see much gain on the city cycle.

As for not feeling the power loss, my only theory is that the throttle-by-wire is already hampering the "feel" of the Hemi so that the marginal horsepower losses from running 87 octane instead of 89 are not always that noticeable in a seat of the pants comparison.

Cruising on the highway, the WK2 might require, say, 30hp to maintain 65mph. The Hemi can generate 30hp easily without requiring the 89 octane for that particular situation which then can arguably yield an economy uptick. However, once you need to many climb steep grades or tow a trailer, a lot more demand is placed on the Hemi and if it's running 87 octane, even with the higher energy content in 87, the reaction to detonation may cause enough timing pull to yield less fuel economy than simply running the 89. I'd like to see a Mythbusters show on this.

__________________An expert is one who knows more and more about less and less until he knows absolutely everything about nothing.
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2013 GC Ltd Hemi (2nd Jeep)

Yep - count me as skeptical the more I think about it. If Jeep could get better MPG with 87 I'm sure they would exploit that to the hilt. All auto makers are doing whatever they can to get better MPG ratings to sell vehicles and also for overall fleet MPG requirements. There must be other factors involved...or error...

This engine is designed to meet all emissions
regulations and provide satisfactory
fuel economy and performance when using
high-quality unleaded gasoline having
an octane range of 87 to 89. The manufacturer

recommends the use of 89 octane for

optimum performance. The use of premium gasoline is
not recommended, as it will not provide any benefit over
regular gasoline in these engines.